EFFECT OF ANIONS OF LOW MOLECULAR WEIGHT ORGANIC ACIDS ON ADSORPTION AND DESORPTION OF ALUMINUM BY AND FROM A KAOLINITE AT DIFFERENT PH

The toxicity of aluminum ions is the main cause of the unfavorable effect of acid soils on plants. The extent of this toxicity depends on a variety of factors, including organic acids. In the present paper, the effect of anions of four low molecular weight organic acids on the adsorption and desorption of Al by and from a kaolinite was investigated. The results showed that the presence of organic anions led to a decrease in the adsorption of Al. The ability of organic anions in hindering Al adsorption followed the order: Oxalate ? malonate "> malate "> lactate. The adsorption increased with the increase in pH in the lactate system and the control system, whereas an opposite trend occurred in the other three anion systems. The presence of organic anions also induced a decrease in the desorption of Al. The proportion of desorbed Al to adsorbed Al was higher than that in the control system. Both the adsorption and the desorption decreased with the increase in concentration of organic acid, with the difference between adsorption and desorption larger at low concentrations. The difference in effect among the four anion species was interpreted in terms of differences in their nature, with the dissociation constant of the anions and the stability constant of the Al-complexes as the parameters.